In order to elucidate a molecular mechanism of cell membrane fusion, membrane fusion in phospholipid membrane systems (vesicle, monolayer and bilayer membranes) induced by divalent cations, temperature and osmotic pressure gradient will be examined for the cases with and without proteins incorporated in the membranes. The membrane fusion will be correlated with the degrees of free-energy increase of the membrane and dehydration of bound water from membrane surface for the case of divalent ion-induced fusion, and with the degree of membrane expansion for the case of temperature or osmotic gradient induced membrane fusion. Also, in order to apply the membrane fusion in lipid membrane systems to biological membrane systems, the interaction and fusion of synaptic vesicle with phospholipid membranes will be examined with variation of divalent ion concentration. The physicochemical characterization and fusion capability of membrane vesicles derived from erythrocyte membranes will be studied in order to examine the lipid membrane theory for membrane fusion. Methods to be used for these experiments will be two fusion assay (fluorescence and luminescence) techniques, light scattering measurements, observation under phase contrast/fluorescence microscope, NMR technique, Electron microscopy, freeze-fracture Em, Dynamical light scattering and other physico-chemical techniques to determine some properties of membrane molecules.